Sensitivity of adult male Djungarian hamsters (Phodopus sungorus sungorus) to melatonin injections throughout the day: effects on the reproductive system and the pineal.

Twenty-four groups of adult male Djungarian hamsters (Phodopus sungorus sungorus) housed on 16L:8D received daily injections of melatonin, each group at a different hour of the day. Injections (5 micrograms/0.1 ml ethanolic saline 1:10, s.c.) continued for 15 wk, after which the animals were killed and the testes weighed to assess the efficacy of the injections in causing testicular regression. Two periods of melatonin sensitivity were identified. The first was a single time point 5 min before lights on (0455 h). The second was a 5-h period throughout the late afternoon (1600-2100 h; lights out 2100 h). Regression was complete in the 1700- and 1800-h groups only. A few animals failed to respond or regressed very slowly in the 1900- and 2000-h groups, and in the 1600- and 2100-h groups only partial regression was observed after 15 wk of injection. There was no demonstrable effect of exogenous melatonin administration on the endogenous rhythm of pineal melatonin; the rhythm in injected hamsters was of identical duration and amplitude to that in uninjected controls on the same photoperiod.